In drilling wells, a hollow tubular drill string is introduced into the well. Adjacent the bottom of the drill string, a drill tool casing houses a drive motor which rotates a central shaft which projects beyond the drill tool casing and mounts a drill bit. Rotation of the drill bit extends the length of the well as the drill tool casing is advanced incrementally.
The drive motor for the drill shaft may be hydraulic or pneumatic, of the positive displacement (PDM) or turbine type mounted within the drill tool casing. Conventionally, the motor is driven by forcing air or drilling mud through the casing and the motor. Drilling mud may consist of clay, water and/or oil, weighting material such as barium sulfate or hematite, sand, quartz, various types of pulverized, granulated or chips of abrasive material, and chemical polymers. Most frequently, drilling mud is pumped from the surface and through the drill string into the drill tool casing so that it fills the interior of the drill tool casing and the well annulus. The mud is formulated such that it exerts an isostatic pressure which increases on the order of 0.5-1.0 pounds per square inch per foot of depth in the well so that the isostatic pressure may be comparable to subterranean pressure at the bottom of the well in order to prevent well collapse. Thus, at the bottom of a 15,000-foot well, the pressure in the drilling mud may be 15,000 psi. The mud within the drill tool casing is used to drive the fluid motor, and the mud exhausted from the motor is directed through nozzles in the drill bit so as to expel chips and other material disintegrated by the drill bit and also to provide a coolant for cooling the cutting surfaces of the drill bit to improve its cutting efficiency. The pump is normally at the surface adjacent the well, and supplies mud at a pressure sufficient to overcome the pressure drop due to friction of the mud flowing through the drill string. This pressure is further elevated above isostatic pressure so as to drive the motor when its outlet is at isostatic pressure plus the pressure required to exhaust the mud through the nozzles in the drill bit.
The drill bit drills a well hole which is larger in diameter than the outside diameter of the drill tool casing so that an annular space is provided between the drill tool casing and the sides of the well hole through which the casing is advanced during the drilling operation. The drilling mud is exhausted into this annular space and carries with it chips and other drilling debris dislodged by the drill. The added surface hydraulic pressure of the drilling mud forces the spent drilling mud upwardly through the annular space around the drill tool casing and along the entire length of the drill string to the surface for filtration and recycling. Thus, while the pressure gradient in the drilling mud within the well bore provides increasing pressures along the drill string to the lower depths of the well, the pump at the surface supplies the effective operating pressure within the drill casing at the inlet of the fluid motor. While the differential pressure across the motor may represent a pressure loss in the range of 300 to 1500 psi, there is no significant pressure loss along the length of the bore in the rotary drill shaft within the casing. There is another pressure loss of 500-1500 psi through the nozzles in the drill bit and a gradual diminishing of the pressure as the drilling mud flows upwardly around the drill casing and along the remainder of the drill string for recycling to the pump for returning the drilling mud to the fluid motor.
In recycling the drilling mud, the larger particles of drilling debris are filtered from the mud in filters and settling tanks, but the constituents of the drilling mud themselves may still have high abrasive character. Although it is possible to design the fluid motor to withstand the flow of abrasive drilling mud through the motor, the bearing devices which center the rotary shaft within the drill tool casing are simultaneously subject to large fluctuating axial and radial mechanical forcers and severe abrasion by the drilling mud. Conventional long-life sealed lubricated bearings are not available for use in an environment where the pressures may vary from atmospheric to more than 20,000 psi, and where the pressure medium is an abrasive fluid such as drilling mud. Substantial down time and cost is required to withdraw the drilling string from the drill hole for maintenance operations upon the motor, the bearing assembly, and the associated drill bit.
Prior to the present invention, the bearings associated with positive displacement motors and turbines were replaced frequently, and the cost of replacement was considered a necessary expense which must be tolerated in any deep well drilling operation.